OLEDs have become important light-emitting elements in a new flat panel display device because they have advantages of self-luminescence, high contrast, wide color gamut, simple manufacturing process, low power consumption, enabling flexible display and etc.
In a pixel of an OLED display panel, each of sub-pixels includes a driving transistor. In an OLED pixel circuit, current flowing through the OLED is controlled by a data signal Vdata and affected by a threshold voltage Vth of the driving transistor.
Due to a fact that features such as threshold voltages and mobilities of thin film transistors (TFTs) in respective pixel circuits are different, the driving transistors in respective OLED pixel circuits may have different performance parameters accordingly. As a result, the currents flowing through respective OLEDs may be affected by different shifts of the threshold voltages Vth of the driving transistors, and thus be different, so that brightness uniformity and brightness consistence of the OLED display device are adversely affected, thereby display quality of the OLED display device is degraded.
Therefore, a compensation circuit is provided for each pixel circuit. The compensation circuit is connected to a gate electrode of the driving transistor, and is configured to maintain a voltage of the gate electrode of the driving transistor during a light-emitting period. Therefore, it is able for a current flowing through the OLED to be irrelevant to a threshold voltage Vth of the driving transistor, and eliminate the effect on the brightness uniformity and the brightness consistence of the OLED display device by the shifts of the threshold voltages.
However, in the related art, all of the compensation circuits have limited compensation ranges. When the threshold voltage shifts beyond the compensation range, the compensation circuit is invalidated. In a product including a plurality of OLED pixels, the voltage of the driving transistor in each of the OLED pixels has a unique initial value and a unique variation. Thus, when identical compensation circuits are provided for the plurality of OLED pixels respectively, it is possible that some of the compensation circuits are validated for their corresponding OLED pixels, while the other compensation circuits are invalidated for their corresponding OLED pixels. Consequently, the brightness uniformity and the brightness consistence of the OLED display device may be significantly affected.